I feel like a new guy spraying clear!

Ok, I've sprayed spi universal clear for quite a few years now! I've always used one of the older sata rp with a 1.4 tip air pressure cranked at wall and 28 psi at gun! I recently acquired a sata 4000rp with a 1.4! I set the gun up the same as my old one and started having run problems right off the go so I bumped psi at gun to 32, still having runs in overlap areas like corners and such! Any suggestions would be greatly appreciated! The 4000 really lays it slick everywhere else so I really don't wanna go back to my older gun! I've always been known to hang a sag, but always hacked it up to be a fine line between slick and running! I really want to figure out what I'm doing wrong cause I really like the feel and spraying of this gun but I can't have runs like this! Lol Thanks for any help in advance!
 

Chris_Hamilton

Trying to be the best me, I can be
Just watch your overlap more and control your passes more. Where you are getting runs is where you are essentially double coating it due to your passes and overlap in those areas like corners. Just a little more discipline with your passes will solve most of those issues.
Sometimes it happens though. None of us are perfect.:)
 
I'm at 2.75 out and fan 90percent now! Next time I'll try it a 1/4 in at a time! I think all of you are a little right! Discipline wise I've painted for a while so I guess we all get a little too comfortable at times and the 4000 may be more of a hose compared to my old gun hence needing to turn the fluid in a little more!
 

Barry

Paint Fanatic
Staff member
Another thing is that gun is an SCFM hog.
Make sure air at the wall is min 125 first as your adjustments sound close.
 

craig

New Member
Hello All, i know i may get a bit of abuse but i have rattle can sprayed my side fender it looks good but it needs abother coat and i have just ran out, can i just add another layer when i can grab another can??
 

craig

New Member
Thanks its BMW titan silver, got the car cheap and thought whilst COVID was about i would have a go at a bit of home spraying.
 
I understand the psi at the wall makes a difference and I'll check that! But is there an explanation for how the psi at wall changes things when you are regulating it down at the gun?..just curious, like I said I know it does make a difference but just like to learn all I can from the guys that's more experienced than me!
 

Todd Gatman

New Member
I understand the psi at the wall makes a difference and I'll check that! But is there an explanation for how the psi at wall changes things when you are regulating it down at the gun?..just curious, like I said I know it does make a difference but just like to learn all I can from the guys that's more experienced than me!
A little late to the party, so...apologies and sorry for what is a long answer but this is something I have to deal with all the time in my job.

Nothing flows without a change in pressure. Pressure and flow are closely related. The greater the change in pressure, the greater the flow (air is a little bit of an exception because there is a pressure drop beyond which you don't get more flow (google critical pressure)). The 30 psi you adjust at the gun is the pressure (and therefore flow) you are allowing at the outlet of the regulator when the trigger is squeezed. This also happens to be the inlet to the spray gun. We want to control the amount of air into the gun so this is a great place for it.

If you have 120psi at the wall, you would also like to have 120 psi in the hose just before the regulator such that when you squeeze the trigger, the pressure drop across the regulator is high (120psi -30psi=90psi) and therefore you have good flow across it and into the gun due to the nice change in pressure. Unfortunately hoses have friction, fittings have restrictions and you do not get the same pressure at the end of the hose when air is flowing as you had at the beginning. Hose diameter influences pressure drop too. Smaller diameter hoses loose more pressure than large ones. If you were using a ridiculously long, ridiculously small hose, it could lose 90psi from start to finish when you pull the trigger. In this case, you would only get 30 psi in the hose just before the regulator when everything was flowing. Pulling the trigger in this case would produce no change in pressure across the regulator and you would get no flow at all. You would get a big initial puff until things equalized though. Large hoses also contain more air than smaller ones. This means that when you do make that nice pressure drop by using the spray gun you literally have more air available to deliver in a large hose as compared to a small one.

So in summary, you want to use a large hose with a bunch of air in it to carry as much pressure as you can all the way to where you need it.
 

Chris_Hamilton

Trying to be the best me, I can be
Agree with what you are saying, but I will add that the Industry standard type hose in a booth, 35 feet long, 3/8 ID, 1/4 NPT inlet and outlets, and high flow fittings is all that is needed to operate typical automotive refinishing spray guns whether they are HVLP or compliant reduced pressure guns. Anything larger is unnecessary and very troublesome to try and move around with you when you are painting. Other types of specialized industrial spray equipment may have different needs but for what we are doing, that type of hose I described is all that is necessary.
Equally important is a well designed wall piping system and a compressor capable of delivering the CFM's needed. That is something that often gets overlooked by someone doing this as a hobby.
 

orangejuiced86

Garage hack at night.....
I've given up on UV clear. I use to spray it like a champ, no runs ever. Now the stuff just ends up on the floor for me. I did fix some of my issues, but in the end I just moved over to Euro 4:1:1.5 and I'm enjoying painting again, lol.
 

El_Duderino

Member
I use a 5000 Rp 1.2. first coat with needle approximately 1.5 turns out from closed. Psi around 34, all depending weather. Next coat, open the needle enough to wet that coat and so on. 1.4 is too much for HS clears IMHO. I was taught to spray with a 1.4 WFO and took long time to accept that was for old paints. HS clears work much better with smaller tips for me. And the biggest thing I found was starting first coat with needle turned out just enough to spray a wet coat, avoiding heavy wet coats at all costs. A bit of texture cuts and buffs much easier than sags, runs and wave.
 

Bossed

Member
Interesting topic. I enjoy reading about real life experiences from folks who've been there and done it. Allows me to maybe pick up a few pointers.
 
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